In the field of circuit protection, particularly in the automotive industry, it has traditionally been the case that an electronic circuit constituting a load, for example an audio entertainment system, coupled to a power supply system of an automobile has been protected by metallic fuses employing so-called “fuse wire” or metal that bridges a pair of contacts and melts when a current flowing through the wire exceeds a maximum current rating associated with the metal. Such fuses are provided in disposable packages that can be plugged and unplugged into a fuse board within a vehicle, for example so-called “blade” fuses. In this respect, such disposable fuses, in addition to being environmentally unfriendly, require the express provision of the fuse board within the vehicle that has to be easily accessible in order to replace one or more fuses; this has a limiting effect upon automotive design. Furthermore, the need to replace fuses is inconvenient and can result in engineering time to replace the fuses, resulting in higher maintenance costs for the vehicle as well as an increased cost in terms of consumables to an owner of the vehicle.
A resettable or retriggerable “fuse” has therefore been proposed as an alternative to traditional disposable metallic fuses. In essence, the retriggerable “fuse” is, in fact, a circuit providing current shut-off functionality in over-current situations. Such retriggerable “fuse” circuits, known as protected relay circuits, typically employ an n-channel FET as a way of protecting a load against short-circuits. However, mechanical implementations that use mechanical alternatives to the n-channel FET exist as well.
One particularly important requirement of protected relay circuits is that they must be able to provide protection to a load in a number of scenarios. For example, in addition to when the engine of the vehicle is running, the protected relay circuit needs to be in an active state and able to provide protection when the engine of the vehicle is not running, i.e. when the battery is not being recharged and power is not being provided by an alternator of the vehicle. Of course, vehicle manufacturers naturally place constraints upon power consumption by electronic devices in the vehicle when the engine is not running as the charge of the battery must not be unnecessarily drained. A number of loads in the vehicle need to draw small amounts of current, in the order of a few tens of micro-Amps, whilst the engine is not running, for example: an electronic clock and a central locking, alarm and engine immobiliser system, to name but a few.
In relation to certain electronic equipment in the vehicle, the operative state of the equipment can change, for example through user interaction, resulting in the circuits of the equipment drawing greater amounts of current (for example, up to a few Amps) without warning. The central locking, alarm and engine immobiliser system and audio (and possibly audiovisual) entertainment equipment in the vehicle are examples of such equipment.
However, since flow of a load current drawn by the equipment to be protected is relatively high, the protected relay circuit used has to have a relatively low “on” resistance, for example a low Drain-Source “on” resistance (RDSon) for FET implementations, in order to reduce power dissipating by the protected relay circuits. Additionally, such protected relay circuits require a constant bias current, which is unacceptably high, in order to switch the protected relay circuit to an “on” state and provide the necessary protection against over-current events.
Further, in order to achieve the low RDSon mentioned above, n-channel FET-based protected relay circuits require a charge pump, or periodic gate refresh circuitry. However, the provision of the charge pump or periodic gate refresh circuitry requires the protected relay circuit to draw an unacceptably high level of current in the order of at least 100 μA. As will be appreciated, repeated instances of the n-channel FET protected relay circuit to replace all or almost all fuses in the vehicle that protect the various loads contained in the vehicle with known high-side FET protected relay circuits will result in an unacceptably high current drain on the battery of the vehicle.
In contrast, p-channel FET implementations of the protected relay circuit can have reduced current requirements to drive a gate of the FET, but have higher financial costs associated with their use (due to increased die area requirements over n-channel implementations) and also require an unacceptably high bias current to provide the necessary protection required. Consequently, p-channel FET implementations are not widely employed. Also, most integrated circuit technologies and techniques are optimised for circuits employing power n-channel FETs. Additionally, p-channel FET implementations are difficult to design to achieve levels of protection comparable to those of n-channel FETs in terms of accuracy of threshold implementation for short-circuit protection.
As can be seen from the above-described known technologies, in some circumstances protected relay circuits consume unacceptably large amounts of current to provide accurate over-current protection and so are sub-optimal implementations from the point of view of vehicle manufacturers.